1. Field of the Invention
The invention relates to a firing mechanism for a subterranean well perforating gun, and particularly to a firing mechanism of generating high detonating energy.
2. History of the Prior Art
The perforation of a well casing and the adjacent production formation is a procedure employed in practically all subterranean oil and gas wells. In the completion of wells of substantial depth, it has become a common practice to employ a perforating gun that is run into the well on the bottom of a tubing string. A packer is incorporated in the tubing string and is set in the well casing so as to position the perforating gun adjacent the production formation. This practice has the advantage of permitting a much larger perforating gun to be employed than is possible if the gun were run into the well through a tubing string on a wireline. More importantly, it permits the perforating of the well in the so-called "underbalanced" condition wherein the fluid pressure existing in the tubing string adjacent the formation is substantially less than the anticipated fluid pressure of the production formation after the perforating operation is completed. This permits a relatively high velocity flow of production fluid from the newly formed perforations in the production formation thus flushing the perforations of the debris that is commonly associated with a perforating operation.
One negative factor encountered in mounting of a perforating gun on the bottom of a tubing string is the high cost involved in replacing the gun in the event the gun fails to fire. The failure of perforating guns to fire is particularly prevalent when the production formation involves ambient temperatures of 400.degree. F. or higher and wherein the perforating gun and its firing head must be exposed to such temperatures for relatively long time periods, such as a hundred hours or more, while other operations are being performed in the upper portions of the well or at the well head. In the first place, very few explosives commonly used in perforating guns are capable of maintaining thermal stability at such temperatures and for such a long period of time. This problem may be cured, however, by utilizing a different type of explosive, hereinafter called a thermal resistant explosive, as the explosive element of the shaped charges normally employed in a perforating gun.
The utilization of such thermal resistant shaped charges immediately raises another problem, namely, such thermal resistant shaped charges require very high detonating energy in order to effect their detonation. The detonating energy produced by an impact detonatable primer, generally comprising lead azide as the explosive element, is not sufficient to institute the high energy detonation of a booster charge or primer cord formed of thermally stable explosives. Hence, the primer may fire, but the booster charge and/or primer cord leading to the shaped charges of the perforating gun will not be detonated or, if detonated, the level of detonating energy will be too low to effect the detonation of the shaped charges of a thermal resistant explosive.